# Speed/length Ratio

Discussion in 'DIY Marinizing' started by muddin redneck, Jun 24, 2009.

1. Joined: Jun 2009
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### muddin redneckDO IT IN THE MUD!!!

what is the speed length ratio? is the spped length ratio even real? if so how does it relate to planing a boat? i am wanting to know how to figure it so i know if i go with a small motor on my boat it will still plane out

2. Joined: Oct 2008
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Location: Japan

Speed length ratio is all about waves and hence wave making resistance and how this affects the size of boat in relation to its speed.

In the 'old' days it was called Taylor Quotient, some still use this. This was simply speed in knots divided by square root, in feet, of waterline length.

It is more commonly referred as Froude number. these days, and is non-dimensional...ie unitless.

So, depending upon the length of your boat and the amount of power it has, will depending upon a curve like that attached.

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3. Joined: Nov 2003
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### PARYacht Designer/Builder

Simply put the S/L ratio is boat speed in knots, divided by the square root of the LWL (in feet).

It's a figure used to determine speed potential. For example, your planning powerboat, with will have a S/L over 2.5. It's one of several "numbers" that we'd use to evaluate speed potential.

It would be easier if you told us what boat you have (make, model and year), the engine you currently have, the speed it presently runs at WOT, plus what you wish to hang on it (new motor) and we'll have enough information to work with to tell you what speeds you can expect.

4. Joined: Jun 2009
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### muddin redneckDO IT IN THE MUD!!!

i have a 16' bass tracker it is 48" wide and im told it has a coast gaurd bow. i currently have a 70 hp johnson on it (and its only rated for a 60hp). gps speed is 30-32 mph depending on how choppy the river is. but i have trouble getting to the fishing spots i like ie. back waters of the mississippi river and i was want to put a surface drive motor on it or a long tail motor. im a welder so im gonna build it myself. i was hoping to ba able to go with about a 16hp honda motor and i still dont know what kind of prop size and pitch to get

5. Joined: Nov 2003
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### PARYacht Designer/Builder

Bass Tracker sells a lot of different makes and models.

Assuming your boat is about 1,500 pounds and the typical welded garvey/jon/center console with modest V, she'll run about 20 knots (maybe slightly better with a light load) on a 16 HP Honda. The boat would like a 20 HP better, so it can hop up on plane easily, particularly with a load and rough conditions.

Make, year and model would help refine the top speed considerably. This would pin down the shape and weight of the boat.

6. Joined: Jun 2009
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Location: Muscatine, Iowa

### muddin redneckDO IT IN THE MUD!!!

the model is bass tracker 3. 1982 is is the year its 16' long 54" wide. the coast guard tag says its a 4 person 572lbs. total weight is 945 with gear motor and people. rated for a 60 hp motor. this is all the info i can find it come from the title and rating plate. for the 16 hp or 20 hp motor how do i figure out my prop size and pitch and do you know a good place to get a 2 bladed prop designed for the mud and shallow water
thanks again for all your help

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Location: Noosa.Australia where god kissed the earth.

### pistnbrokeI try

a quick google on mud motor will bring up all the american suppliers and you can study the designs...spare parts lists show the props for each motor ..few phone calls and its all done ...get the welder out but your design must be based on if you are going to use a right or left hand prop ie chain or gear reduction or direct drive ......

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